Monotectoid

The Al-Zn system was the first studied extensively in an attempt to verify the theory for spinodal decomposition [24], The equilibrium diagram for this system, shown in Fig. 18.12, shows a monotectoid in the Al-rich portion of the diagram. The top of the miscibility gap at 40 at. % Zn is the critical consolute point of the incoherent phase diagram. 

A number of binary systems will form an AB solid solution at higher temperatures because of the entropy of mixing term, but will become immisdble at lower temperatures, segregating into ai, an A-rich AB solid solution and ol, a B-rich solid solution. In some cases a monotectoid reaction, U2 ai + p, will also occur. Examples include Al-Zn, Hf-Ta, Nb-Zr, and Nb-U. The monotectoid reaction in the Al-Zn system is shown in Eigure 12.26. 

Al-Zn system. The monotectoid reaction occurs at 277°C with a composition of 59 At% Zn. The AI2 is a solid solution of A1 and Zn that is richer in Zn. These two components form a solid solution above 361.5°C up to 60 At% Zn, but below that temperature, they will segregate into a Zln-rich phase and a Zn-poor phase. (Reprinted from Massalski, T.B., Handbook of Binary Alloy Phase Diagrams, ASM, 1990. Reprinted with permission of ASM International. All rights reserved.)... 

Monotectic reactions, in which a liquid decomposes into a solid and a second liquid, can occur in systems that have excess heats of mixing in both the solid and liquid phase. Monotectic systems also have a region of liquid phase immiscibility. Unless done very rapidly, attempts to solidify through this two-phase liquid region will result in almost complete phase separation because of interfacial effects. Monotectoid reactions are also possible in which an AB solid solution separates into an A-rich solid solution and a B-rich (3 phase. 

The phase is shown in fig. 28.25 between the limits 55 mole % LaOi.5 and 66.7 mole % LaOi.5, but its 0 (La+U) ratio is constant at 2.00, which implies a fully-occupied fluorite-type lattice. What makes this possible is the quite rapid decrease in the mean uranium valency at piOi) = 1 atm., T = 1000°C, with decreasing lanthana content below 66.7 mole %, i.e. in the phase-field of RIII, and this is shown in fig. 28.26. The RIII phase transforms into the fluorite-type phase at 1220 10°C at the monotectoid composition of 57.2 mole % LaOi.5 the reverse transformation is significantly faster at 1100°C than in the case of the RII phase. 

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